1. Field of Invention
The invention relates to a shift control device for an automatic transmission mounted on a vehicle. More specifically, it relates to a shift control device performing coast down shift by changing over between a frictional engagement element of disengagement side and a frictional engagement element of engagement side, that is to say, a clutch to clutch shift.
2. Description of the Related Art
An automatic transmission has a first shaft and a second shaft arranged in parallel with each other. A main transmission mechanism is arranged on the first shaft and a auxiliary transmission mechanism is arranged on the second shaft. The main transmission mechanism achieves three forward ratios and one reverse ratio. The auxiliary transmission mechanism achieves three forward ratios. The automatic transmission achieves multistage, for example, five forward ratios, by the combination of the ratios of the main transmission mechanism and the auxiliary transmission mechanism.
In the automatic transmission, a down shift is performed when an accelerator pedal is released and a foot brake is operated, that is, a coast down shift is performed. For example, when the automatic transmission is shifted from the third ratio to the second ratio, a fourth brake of the auxiliary transmission mechanism is disengaged and a fifth brake of the auxiliary transmission mechanism is engaged. In this state, the main transmission mechanism is kept in a predetermined gear ratio (for example, second ratio) wherein a frictional engagement element for coast state is engaged. The frictional engagement element for coast state is, for example, a first brake arranged in parallel with a second brake interposed with a one-way clutch in series.
In the aforementioned automatic transmission, when the coast down shift is performed, especially at a high vehicle speed, as shown in FIG. 10, a hydraulic pressure of engagement side is increased and thus the fifth brake is engaged, and a hydraulic pressure of disengagement side is placed in a state for disengaging. Therefore, an input rotation speed is fairly increased. As a result, a momentary big engine brake feeling is incurred based on a difference between the input rotation speed and the engine rotation speed, and a driver may be uncomfortable. On the other hand, when the coast down shift is performed at a low vehicle speed wherein the engine brake is not achieved, but the down shift is late. Therefore, when an acceleration is required again, a time lag occurs because of the lateness of the down shift. In this case, even when the automatic transmission is in the coast down state, in case where the deceleration rate is small, the automatic transmission is in a driven (negative drive) state wherein a torque is transmitted from the vehicle wheels to the engine. On the other hand, in case where the deceleration rate is large, a input shaft rotation speed becomes less than the engine idle rotation speed, and the automatic transmission is in a drive (positive drive) state wherein a torque is transmitted from the engine to the vehicle wheels. At that time, the hydraulic pressure for the first brake as the frictional engagement element for coast state is kept at a pressure for engagement. Therefore, the main transmission mechanism is kept in the state of the second ratio irrespective of the drive state or the driven state because of the engagement of the first brake.